Leaving Car Reminder Alert

Have you tried looking into what Shortcuts can do, this article gives some pointers and I notice it mentions control of system level features such as WiFi and Bluetooth, may be some link can be created that monitors there change of status (just guess though).

http://www.iphonehacks.com/2018/09/cool-things-siri-shortcuts.html

You could be cheeky and try sending an email to the Editor-in-Chief of MacStories, Federico Viticci, who I believe is quite proficient with respect to Shortcuts and simply ask the question if he believes such a shortcut is possible.

Ok post your feedback to to Apple

https://www.apple.com/feedback/

As I understand it he wants an alert to be triggered as soon as the device detects a Bluetooth disconnect as you would likely get on turning off the ignition, but he would still be in the vehicle when that happens. The alert is to remind him not to forget iPhone before he leaves the vehicle.

How would you hear it? I mean if you left the car the iPhone alerts will go off in the car?

I've never used Shortcuts (or Workflow) before but from what I've read it appears to take a building block approach to putting together various "actions" that when complete can carry out an App based task or tasks.

Use the Shortcuts app on your iPhone or iPad - Apple Support

Shortcuts User Guide - Apple Support

Must admit I'm intrigued enough to also give it a try to see what Shortcuts can do, so I've download App and currently working my way through the above Apple guide, whether it will do what you require only time will tell but you may find an alternative way while messing around and learning.

You may find this helpful.

https://www.macstories.net/stories/ios-12-the-macstories-review/9/#the-shortcuts-app

Happy learning.